The present invention deals with a process control sensor system. More specifically, the present invention deals with a sensor system having multiple sensors and an output device which provides a non-hysteretic output signal with a continuous first derivative in the region where a transition is made between two sensor signals.
Typical autoranging instruments are configured to sense a parameter and provide an output signal indicative of the sensed parameter as the parameter changes over a broad range of values. One such typical autoranging instrument is a digital voltmeter. A digital voltmeter typically has two or more sensor configurations which may include two or more different sensors or a single sensor which can be reconfigured to have two or more gain values. The sensor system provides an output signal indicative of sensed voltage. However, the two or more sensor configurations are used for different ranges. In other words, if the sensed voltage is, for instance, in a range of 0 to 10 volts, a low range sensor configuration is used to provide an output signal indicative of the sensed voltage. However, as the sensed voltage increases in value such as above 12 volts, the total error in the output signal of the first sensor configuration increases rapidly and the output signal from the first sensor configuration eventually becomes less accurate than desired.
Therefore, a second higher range sensor configuration is provided which provides an accurate output signal as the voltage increases. By the same token, however, the second sensor configuration does not provide an output signal which is as accurate (as a percent of the reading) as the voltage decreases below, for example, 10 volts, as does the lower range sensor configuration.
Such prior autoranging instruments are configured to provide an output from the first sensor configuration until the voltage exceeds the preset maximum range value (such as 12 volts). Then, the instrument is configured to abruptly switch the second sensor configuration into the output circuit and provide the system output signal based on the sensor signal from the second sensor configuration as the voltage continues to increase. However, when the voltage decreases such that it falls below a decreasing value (such as 10 volts) the autoranging instrument is configured to abruptly switch back to the first sensor configuration. Thus, such prior systems typically operate with hysteresis in the transition range.
This abrupt switching, and also the hysteresis, typically causes a small shift in the output signal from the instrument due to different calibration errors and non-linearities in each range. This shift in the output and the induced hysteresis provides unacceptable performance in other applications, such as in a pressure transmitter which is used in a precision proportional-integral-derivative (PID) process control loop. The abrupt range change transient causes a large derivative input to the PID control loop which is problematic.
The process control industry has also developed sensors, such as pressure sensors, which provide a highly accurate output signal over different ranges. For example, one such sensor has an upper range limit of 4,000 psi and provides a high level of performance over a calibrated range of approximately 30 to 1 (or from approximately 150 psia to 4,000 psia). However, as the 4,000 psia sensor ranges down below 150 psia, the total error (as a percent of reading) in the output signal increases rapidly and provides a performance level which is less than desired. Other sensors are used to measure pressure ranging from 0 to 200 psia and have been designed to provide a high level of performance over this entire operating range. However, the sensors cannot sense pressures accurately over this upper range limit. The industry has encountered difficulty in smoothly transitioning between the two sensors. As discussed above, this causes problems in PID control loops.
Various analog or digital filtering techniques can be used to compensate for the shift in output during the range transition. However, such techniques often result in a phase lag that can affect the control loop. Also, the resulting hysteresis cannot be filtered out.